The RF device usually comprises a transceiver itself comprising a transmitter TX (circuit or device) and a receiver (circuit or device) RX both driven by local oscillator signals which are output by a dedicated generator. As is well known by the man skilled in the art, in a direct conversion architecture the transmitter comprises:                an I/Q modulator fed by the baseband processor and outputting (I+, I−) and (Q+, Q−) baseband (modulated) differential signals having an orthogonal phase relationship,        two paths dedicated to I and Q differential signal processing respectively and each comprising an automatic gain control means fed with the baseband I (or Q) differential signals and delivering baseband I (or Q) differential signals with a chosen gain, and a mixer for mixing the baseband I (or Q) differential signals, delivered by the automatic gain control means, with local oscillator carriers (such as sine and cosine RF carriers) output by a generator and having a frequency FLO, in order to convert them into RF signals having a carrier frequency equal to the local oscillator frequency FLO,        an adder for adding the RF signals, and        a power amplifier for amplifying the RF signals output by the adder and feeding an antenna.        
“I signal” is meant to be understood here as an in-phase component signal and “Q signal” is meant to be understood here as a quadrature component signal.
Each automatic gain control means comprises a low-pass filter for filtering the baseband I (or Q) differential signals and applying a chosen first attenuation to each of them in order to decrease their amplitudes, and a differential transconductor for applying a chosen second attenuation to each differential signal coming from the low-pass filter in order to decrease its continuous component (DC).
By varying the combination of the first and second attenuations it is possible to control each path gain. Therefore, the transmitter may output RF signals with a gain chosen in a range, which is for instance equal to 40 dB when the communication network is of the GPRS or EDGE type.
In a direct conversion architecture the baseband I or Q differential signals are directly modulated by the local oscillator (LO) carrier in the mixer. Now, the modulator, being imperfect, introduces a DC offset between the baseband I or Q differential signals it receives, i.e. between I+ and I− or between Q+ and Q−, which necessitates the LO carrier rejection (or leakage) in order for its contribution to be as low as possible.
Some communication networks, such as the GPRS or EDGE ones, impose on the LO carrier that rejection stay below a threshold over the whole gain range. This requirement cannot be satisfied when using a state-of-the-art transmitter. Indeed, when the threshold is equal to −33.2 dB, the requirement is satisfied as long as the introduced attenuation remains smaller than approximately 24 dB, which is far from the maximum attenuation which is 40 dB in the GPRS or EDGE network.
In fact the LO carrier rejection becomes greater than the threshold when the transconductor generates its maximum second attenuation, i.e. when every one of its signal copy cells is used, while at the same time the first attenuation generated by the low-pass filter keeps increasing. In this case, the DC offset between the baseband differential signals (for instance I+ and I−) remains constant while at the same time the amplitude of the I+ or I− decreases. Therefore, the LO carrier rejection keeps increasing.